An EPLD-based time-of-arrival acquisition board for neutron coincidence counting

A PC acquisition board for a new generation of passive assay neutron multiplicity counters is described. The board acquires the time-of-arrival of each digital pulse from a set of neutron counters together with the relevant detector identification number and stores these data in the PC memory. By statistical processing of the data safeguards verifications of plutonium bearing materials or nuclear wastes characterizations can be done. Simultaneous events from different detectors are processed sequentially (derandomized) without any data loss. A FIFO (First In First Out) bank on the board makes it possible to acquire data at a peak rate much higher than the PC bus bandwidth. All the logic circuits (input section with sequencer and channel encoder, 32-bit system clock, FIFO controller and PC bus interface) are implemented with EPLDs (Erasable Programmable Logic Devices), making it possible to fit up to 64 input channels in an AT full size board and up to four boards in the PC bus. As a general rule the processing time increases with the number of input channels; exploiting the internal architecture of the EPLDs several solutions with different trade off between speed and number of inputs were analyzed. The 36 input, 100 ns speed version was built, fully characterized and integrated in a multiplicity counting system. With an assembly driver up to 0.5 Mevents/s can be recorded with a 286 or higher based PC; the total number of events that can be continuously collected at maximum speed is limited only by the size of the PC extended memory. The board is also suited for time correlation analysis in other fields e.g. photon counting